1. Technical Field
The present invention relates to a three-plate projection-type image display apparatus including light valves (e.g., liquid crystal panels), one each for red, green and blue light beams, as a modulation means so that display images of the respective light beams are combined in the apparatus and projected to form a magnified image on a screen.
2. Background Art
The projector market, especially for projection-type image display apparatuses using a transmission-type liquid crystal panel, now is growing rapidly. The trends of products can be divided into two major categories: higher brightness and smaller size. In particular, the diagonal size of an effective aperture of a liquid crystal panel is reduced from 1.3 inches, which has been a mainstream diagonal size, to 0.9 inches at present and is expected to be reduced further in the future. While reducing the effective aperture size, the transmission-type liquid crystal panel has a very small black matrix (BM) and a numerical aperture high enough to be comparable with that of a conventional liquid crystal panel that is one size larger than the above liquid crystal panel. With the implementation of such a small-size high-density liquid crystal panel, a color combination portion for combining display images on the liquid crystal panels also needs to provide higher accuracy.
Next, the configuration of a conventional projection-type image display apparatus using liquid crystal panels will be described. Three-plate projection-type image display apparatuses including liquid crystal panels, one each for red, green and blue light beams, can be classified roughly into two categories according to their characteristics in color combination: a cross-prism system and a mirror-sequential system. FIGS. 7 and 8 schematically show the basic configurations of conventional projection-type image display apparatuses employing the cross-prism system and the mirror-sequential system, respectively. The following is an explanation for each of the configurations.
As shown in FIG. 7, a cross-prism projection-type image display apparatus 100 includes a light source portion 101, a color separation optical system 102, a relay optical system 103, a light valve portion 104, a color combination optical system 105, and a projection optical system (a projection lens) 106.
The light source portion 101 includes a light source 107 and a reflector 108. The light source 107 forms an arc by discharge between electrodes to generate a randomly polarized light beam. The reflector 108 reflects the light beam from the light source 107 in one direction along its axis of rotational symmetry.
A light beam from the light source portion 101 enters a blue-reflection dichroic mirror 109 of the color separation optical system 102, where a blue light beam of the incident light is reflected. Then, the blue light beam is reflected from a total reflection mirror 110 and passes through a condenser lens 111 into a blue light valve unit 112. Green and red light beams are transmitted by the blue-reflection dichroic mirror 109 and enter a green-reflection dichroic mirror 113, where the green light beam is reflected and passes through a condenser lens 114 into a green light valve unit 115. The red light beam is transmitted by the green-reflection dichroic mirror 113 and enters the relay optical system 103. Then, the red light beam passes through an entrance lens 116, a total reflection mirror 117, an intermediate lens 118, a total reflection mirror 119, and a condenser lens 120 into a red light valve unit 121.
The light valve portion 104 includes the blue, green and red light valve units 112, 115 and 121, which are arranged in accordance with the respective light beams. Each of the light valve units 112, 115 and 121 includes an entrance polarizing plate 122, a liquid crystal panel 123, and an exit polarizing plate 124, as shown in FIG. 2. The entrance polarizing plate 122 is rectangular in shape and designed, e.g., to transmit light polarized in the short side direction and to absorb light polarized in the direction perpendicular thereto. The light beam passing through the entrance polarizing plate 122 enters the liquid crystal panel 123. The liquid crystal panel 123 has many pixels arranged in the form of an array and can change the polarization direction of the incident light at each pixel aperture with an external signal. In this example, when the pixels are not driven, the liquid crystal panel 123 transmits the incident light while rotating its polarization direction by 90 degrees; when the pixels are driven, the liquid crystal panel 123 transmits the incident light without changing its polarization direction. The exit polarizing plate 124 has polarization characteristics in the direction perpendicular to the entrance polarizing plate 122. In other words, the exit polarizing plate 124 has a transmission axis in the long side direction of its rectangular outline and transmits light polarized in this direction. Therefore, the light beam that has entered the undriven pixel of the liquid crystal panel 123 and been transmitted with its polarization direction rotated by 90 degrees can pass through the exit polarizing plate 124 because it is polarized in the direction parallel to the transmission axis. On the other hand, the light beam that has entered the driven pixel of the liquid crystal panel 123 and been transmitted with its polarization direction unchanged is absorbed by the exit polarizing plate 124 because it is polarized in the direction perpendicular to the transmission axis.
The light beams thus transmitted through the light valve portion 104 enter the color combination optical system 105. The color combination optical system 105 is a color combination prism formed by joining four triangular prisms so that a blue-reflection dichroic mirror surface 125 and a red-reflection dichroic mirror surface 126 cross at right angles. The blue and red light beams incident on the color combination optical system 105 are reflected from the blue-reflection dichroic mirror surface 125 and the red-reflection dichroic mirror surface 126, respectively, and then enter the projection lens 106, which acts as a projection optical system. The green light beam passes through the blue- and red-reflection dichroic mirror surfaces 125, 126 and enters the projection lens 106.
The projection lens 106 magnifies and projects the incident light onto a screen (not shown). In this manner, images of three light beams, each of which is formed in the light valve portion 104, are combined and displayed as a color image.
As shown in FIG. 8, a mirror-sequential projection-type image display apparatus includes a light source portion 201, a color separation optical system 202, a light valve portion 203, a color combination optical system 204, and a projection optical system (a projection lens) 205.
The light source portion 201 includes a light source 206 and a reflector 207. The light source 206 forms an arc by discharge between electrodes to generate a randomly polarized light beam. The reflector 207 reflects the light beam from the light source 206 in one direction along its axis of rotational symmetry.
A light beam from the light source portion 201 enters a blue-reflection dichroic mirror 208 of the color separation optical system 202, where a blue light beam of the incident light is reflected. Then, the blue light beam is reflected from a total reflection mirror 209 and passes through a condenser lens 210 into a blue light valve unit 211. Green and red light beams are transmitted by the blue-reflection dichroic mirror 208 and enter a green-reflection dichroic mirror 212, where the green light beam is reflected and passes through a condenser lens 213 into a green light valve unit 214. The red light beam is transmitted by the green-reflection dichroic mirror 212 and passes through a condenser lens 215 into a red light valve unit 216.
The light valve portion 203 includes the blue, green and red light valve units 211, 214 and 216, which are arranged in accordance with the respective light beams. Each of the light valve units 211, 214 and 216 includes an entrance polarizing plate 217, a liquid crystal panel 218, and an exit polarizing plate 219, as shown in FIG. 2. The entrance polarizing plate 217 is rectangular in shape and designed, e.g., to transmit light polarized in the short side direction and to absorb light polarized in the direction perpendicular thereto. The light beam through the entrance polarizing plate 217 enters the liquid crystal panel 218. The liquid crystal panel 218 has many pixels arranged in the form of an array and can change the polarization direction of the incident light at each pixel aperture with an external signal. In this example, when the pixels are not driven, the liquid crystal panel 218 transmits the incident light while rotating its polarization direction by 90 degrees; when the pixels are driven, the liquid crystal panel 218 transmits the incident light without changing its polarization direction. The exit polarizing plate 219 has polarization characteristics in the direction perpendicular to the entrance polarizing plate 217. In other words, the exit polarizing plate 219 has a transmission axis in the long side direction of its rectangular outline and transmits light polarized in this direction. Therefore, the light beam that has entered the undriven pixel of the liquid crystal panel 218 and been transmitted with its polarization direction rotated by 90 degrees can pass through the exit polarizing plate 219 because it is polarized in the direction parallel to the transmission axis. On the other hand, the light beam that has entered the driven pixel of the liquid crystal panel 218 and been transmitted with its polarization direction unchanged is absorbed by the exit polarizing plate 219 because it is polarized in the direction perpendicular to the transmission axis.
The light beams thus transmitted through the light valve portion 203 enter the color combination optical system 204. The color combination optical system 204 includes a green-reflection dichroic mirror 220, a red-reflection dichroic mirror 221, and a total reflection mirror 222. The blue light beam emitted from the blue light valve unit 211 passes through the green-reflection dichroic mirror 220 and the red-reflection dichroic mirror 221 in sequence and enters the projection lens 205, which acts as a projection optical system. The green light beam emitted from the green light valve unit 214 is reflected from the green-reflection dichroic mirror 220, passes through the red-reflection dichroic mirror 221, and enters the projection lens 205. The red light beam emitted from the red light valve unit 216 is reflected from the total reflection mirror 222 and the red-reflection dichroic mirror 221 in sequence and enters the projection lens 205.
The projection lens 205 magnifies and projects the incident light onto a screen (not shown). In this manner, images of three light beams, each of which is formed in the light valve portion 203, are combined and displayed as a color image.
The above two projection-type image display apparatuses have typical configurations currently used for presentation, and their characteristics will be described below.
The projection-type image display apparatus using the cross-prism system for color combination (FIG. 7) has the advantages that (1) the focal length and size of the projection lens can be reduced because the projection distance between each of the liquid crystal panels and the projection lens is made shorter, and (2) the accuracy can be ensured easily under vibration and shock because the color combination optical system has a small size and the reflection planes are formed of prisms. However, there are problems as follows: (1) when the four prisms of the color combination optical system 105 are not joined together with sufficient accuracy, a vertical line appears on the center of a projection image due to the interface between the prisms; (2) each of the reflection planes 125, 126 of the color combination optical system 105 is formed by arranging two prisms so that a dichroic mirror surface of one prism is flush with that of the other prism, and thus color irregularity is caused if the two dichroic mirror surfaces of each reflection plane do not have the same spectral characteristic; (3) defocus of a projection image, such as a double image, occurs unless the dichroic mirror surfaces of two prisms that form each of the reflection planes 125, 126 are flush with each other without any distortion and deviation; and (4) the relay optical system 103 is needed in addition to the color separation optical system 102, which increases the apparatus size and also leads to color irregularity when the light source or illumination optical system has non-uniform brightness because the light source image of a light beam that passes through the relay optical system is reversed with respect to the light source images of two other light beams that do not pass though the relay optical system. Considering the improvement in accuracy of the color combination optical system that accompanies the use of such a high definition liquid crystal panel described above, the problems (1) and (3) particularly have to be solved. Therefore, it is necessary to enhance machining accuracy of the color combination optical system further.
The projection-type image display apparatus using the mirror-sequential system for color combination (FIG. 8) has the advantages that (1) the apparatus is relatively inexpensive and adapted easily to a large liquid crystal panel, (2) the apparatus can reduce the weight, and (3) in the absence of a relay optical system, the apparatus size can be relatively small and nonuniformity in brightness of the light source portion has little effect on projection images. However, there are problems as follows: (1) since a light beam passes through obliquely placed parallel planes, an astigmatic difference is caused, shifting the position of a focus on a vertical line from that on a horizontal line, which results in a blurred projection image; (2) it is difficult to provide flatness of the dichroic mirror surface formed on a thin glass sheet, which results in a blurred projection image; and (3) an increase in size of the color combination optical system 204 makes it difficult to achieve mechanical strength, to resist an external force such as vibration, and to maintain convergence accuracy. In particular, (1) and (2) are serious problems in promoting small size and high definition of a liquid crystal panel. Thus, the cross-prism system so far has gained mainstream use, though there remain the above problems to be solved.
It is an object of the present invention to provide a projection-type image display apparatus including a novel optical system that can overcome the above problems of various types of conventional optical systems, arising when the apparatus uses small-size high-definition liquid crystal panels.
To achieve the object, the present invention has the following configurations.
A first projection-type image display apparatus of the present invention includes the following: three light source portions for emitting red, green and blue light beams, respectively; a light valve portion for modulating each of the light beams from the light source portions; a color combination optical system for combining the light beams modulated by the light valve portion; and a projection lens for magnifying and projecting the combined light beam. The color combination optical system includes three triangular prisms, each of which has a vertex angle of about 30 degrees (preferably 27 to 33 degrees, more preferably 29 to 31 degrees, and most preferably 30 degrees), and is formed by joining the three prisms together so that the side faces of each prism that form the vertex angle are brought into contact to make the vertex angle of one prism next to that of the other prism. Each of the joining planes between the prisms is provided with a dichroic mirror surface acting as a color selection means. The side face of each prism opposite to the vertex angle is used as an incidence plane for each of the light beams. The side face of the prism arranged at one end of the three joined prisms is used as an exit plane for the combined light beam. The optical path lengths of the respective light beams between the incidence planes and the exit plane are substantially equal to one another.
Instead of the three light source portions, the present invention can employ a light source portion for emitting a white light beam. A second projection-type image display apparatus of the present invention includes the following: a light source portion for emitting a white light beam; a color separation optical system for separating the white light beam from the light source portion into red, green and blue light beams; a light valve portion for modulating each of the light beams from the color separation optical system; a color combination optical system for combining the light beams modulated by the light valve portion; and a projection lens for magnifying and projecting the combined light beam. The color combination optical system includes three triangular prisms, each of which has a vertex angle of about 30 degrees (preferably 27 to 33 degrees, more preferably 29 to 31 degrees, and most preferably 30 degrees), and is formed by joining the three prisms together so that the side faces of each prism that form the vertex angle are brought into contact to make the vertex angle of one prism next to that of the other prism. Each of the joining planes between the prisms is provided with a dichroic mirror surface acting as a color selection means. The side face of each prism opposite to the vertex angle is used as an incidence plane for each of the light beams. The side face of the prism arranged at one end of the three joined prisms is used as an exit plane for the combined light beam. The optical path lengths of the respective light beams between the incidence planes and the exit plane are substantially equal to one another.
According to the first and second configurations, the color combination optical system is formed as a prism block in which three prisms are joined together. This makes it possible to increase mechanical strength, to maintain durability, and to ensure accuracy even if an external force such as vibration is applied after convergence has been adjusted, thus providing an optical system with high reliability.
Moreover, all the reflection planes of the color combination optical system are the side faces of a single prism. Therefore, this configuration can overcome such problems of the cross-prism system that a vertical line (shadow) appears on the center of a screen due to the interface between the prisms, color irregularity is caused by the difference in spectral characteristic between two prism surfaces that form one reflection plane, and defocus such as a double image occurs because the two prism surfaces are not flush with each other.
Unlike the cross-prism system, there is no need to align a surface of one prism with that of the other prism so as to form the same plane for joining. Thus, the cost can be reduced.
Unlike the mirror-sequential system, a chief ray does not pass thorough obliquely placed parallel planes. Therefore, images are not blurred. Since the dichroic mirror surface is formed on the side face of a prism, plane accuracy can be achieved easily and images are not blurred.
The distance between the light valve portion and the projection lens (i.e., a back focal length of the projection lens) can be minimized, thus reducing the size and cost of the projection lens.
The use of glass prisms allows the optical paths in the color combination optical system to be filled with glass, so that the optical path length can be made relatively short (specifically, though it may be longer than the optical path length in the cross-prism system, it is significantly shorter than that in the mirror-sequential system). Thus, the size of the apparatus can be reduced.
In the first and second apparatuses, it is preferable that the three prisms of the color combination optical system are first, second and third prisms that are joined in this order; a first dichroic mirror surface is provided at the joining plane between the first prism and the second prism, and a second dichroic mirror surface is provided at the joining plane between the second prism and the third prism; the exit plane is the side face of the third prism other than the joining plane and the incidence plane; a light beam entering the incidence plane of the first prism passes through the first prism, the first dichroic mirror surface, the second prism, the second dichroic mirror surface, and the third prism in sequence and exits from the exit plane; a light beam entering the incidence plane of the second prism passes through the second prism, is reflected from the first dichroic mirror surface to pass through the second prism again, passes through the second dichroic mirror surface and the third prism, and exits from the exit plane; and a light beam entering the incidence plane of the third prism passes through the third prism, is reflected from the side face including the exit plane to pass through the third prism again, is reflected from the second dichroic mirror surface to pass through the third prism yet again, and exits from the exit plane.
This preferred configuration can facilitate the combination of the three light beams and also make their optical path lengths equal.
In the preferred configuration, it is preferable that both the light beams entering the second and third prisms are s-polarized light with respect to the first and second dichroic mirror surfaces. Moreover, it is preferable that the light beam entering the first prism is p-polarized light with respect to the first and second dichroic mirror surfaces.
This preferred configuration can increase the utilization efficiency of light from the light source.
It is preferable that the light beam entering the first prism is a green light beam.
This preferred configuration can increase the utilization efficiency of light from the light source.
In the first and second apparatuses, it is preferable that the three prisms of the color combination optical system are of the same shape.
This preferred configuration can reduce the cost of the color combination optical system.
In the second apparatus, it is preferable that the light valve portion includes three light valves, one each for the respective light beams; the color separation optical system includes at least two dichroic mirrors and three reflection mirrors, the dichroic mirrors separating the white light beam from the light source portion into the red, green and blue light beams, and the reflection mirrors being arranged in accordance with the three light valves so as to guide the separated light beams to the corresponding light valves; and the optical path lengths of the three light beams between the light source portion and the light valves are substantially equal to one another.
Specifically, it is preferable that the three prisms of the color combination optical system are first, second and third prisms that are joined in this order; the exit plane is the side face of the third prism other than the plane joined to the second prism and the incidence plane; the light valve portion includes first, second and third light valves, one each for the respective light beams; the first, second and third light valves are arranged opposite to the incidence planes of the first, second and third prisms, respectively; the color separation optical system includes at least first and second dichroic mirrors and first, second and third reflection mirrors; the first dichroic mirror separates a third light beam from the white light beam emitted by the light source portion, and then the second dichroic mirror separates first and second light beams; the first light beam is reflected from the first reflection mirror, passes through the first light valve, and enters the incidence plane of the first prism; the second light beam is reflected from the second reflection mirror, passes through the second light valve, and enters the incidence plane of the second prism; the third light beam is reflected from the third reflection mirror, passes through the third light valve, and enters the incidence plane of the third prism; and the optical path lengths of the three light beams between the light source portion and the light valves are substantially equal to one another.
According to this preferred configuration, the color separation optical system does not require a relay optical system. Therefore, the size and cost of the apparatus can be reduced. Moreover, the optical path lengths of the three light beams between the light source portion and the respective light valves are substantially equal to one another. Thus, this configuration does not cause the problem of color irregularity resulting from a reverse of the light source image due to a difference in the optical path lengths, which arises along with the use of a relay optical system. Consequently, high image quality can be achieved.
In the above preferred configuration, the optical axis that goes through the first dichroic mirror and the first reflection mirror may be substantially orthogonal to the optical axis that goes through the first reflection mirror and the exit plane, and thus a chief ray of the white light beam can enter the first dichroic mirror at the angle of incidence smaller than 45 degrees.
Alternately, the optical axis that goes through the first dichroic mirror and the third reflection mirror may be substantially parallel to the optical axis that goes through the first reflection mirror and the exit plane, and thus a chief ray of the white light beam can enter the first dichroic mirror at the angle of incidence larger than 45 degrees.
In the first and second apparatuses, it is preferable that light emitted from the light source portion is polarized light having a uniform polarization direction.
This preferred configuration can improve the utilization efficiency of light from the light source portion. When a liquid crystal light valve is used in the light valve portion, this configuration can reduce optical absorption by an entrance polarizing plate.
In the first and second apparatuses, it is preferable that the light valve portion includes three light valve units, one each for the respective light beams, and each of the light valve units includes at least an entrance polarizing plate as a polarizer, a transmission-type liquid crystal panel, and an exit polarizing plate as an analyzer.
This preferred configuration can form images with a simple structure.
In the first and second apparatuses, it is preferable that the base of each of the triangular prisms is a right triangle.
This preferred configuration can make the optical path lengths of the respective light beams in the color combination optical system equal.